The invention is in the general fields of sensing the angle of a shaft and of controlling equipment on the basis of shaft angle position, and is more specifically in the field of programmable limit switches using resolver (synchro)-to-digital angle conversion.
The measurement of shaft angle is a common requirement in a variety of equipment such as machine and process control systems, because the position of a shaft can be used to activate or deactivate other elements of the system, and/or the rotational speed or acceleration of the shaft can be used to control other events. Many different forms of shaft angle transducers have been developed, such as potentiometers, brush encoders, optical encoders, synchros, and the special form of a synchro called a resolver. Synchros and resolvers are of particular interest because of their relatively high reliability and functional stability, as they do not depend for signal integrity on moving electrical contacts, do not exhibit appreciable aging or wear, do not drift significantly with time, and temperature changes have negligible effect on their performance. Synchros have a rotor connectable to the shaft of interest and rotatable inside a stator. The rotor typically carries one or three windings, and the stator carries three windings connected 120.degree. apart. Typically the rotor winding is excited with an a-c voltage called the reference voltage, and the voltage induced in any stator winding is proportional to the cosine of the angle between the rotor coil axis and the stator coil axis. The voltage induced across any pair of stator windings is the sum or difference, depending on the phase, of the voltages across the two windings. A resolver is a special form of a synchro, and typically has a single rotor winding and a pair of stator windings which are spatially at 90.degree. to each other instead of 120.degree. as in the case of synchros generally. The two-wire rotor terminals accept an a-c reference voltage, and the pair of two-wire output terminals, connected to isolated stator windings, produce a pair of voltages which are at the reference voltage frequency but have amplitudes proportional respectively to the sine and cosine of the spatially angular position of the shaft. The ratio of the two outputs is an accurate measure of the tangent of the shaft angle, and this ratio can be digitized and used as an input to a programmable limit switch system which activates and deactivates limit switches to control process steps or equipment elements. Such programmable limit switches, because of their reliability and easy adaptability to new control functions, are gradually replacing the older type of limit switch systems which rely on cams affixed to the shaft of interest and mechanically coupled to limit switches or other electromechanical elements.
Several schemes for converting the outputs of synchros and resolvers to a digital signal are discussed in a 5-part article: Schmid, H., Synchro-To-Digital Converters, Electronic Design, 6, Mar. 15, 1970, pp. 178-185, 7, Apr. 1, 1970, pp. 50-58, 8, Apr. 12, 1970, pp. 76-79, 9, Apr. 26, 1970, pp. 72-77 and 10, May 10, 1970, pp. 98-103. A relevant proposal is discussed in the part in Electronic Design, 8, and involves connecting the outputs of a resolver selected for the proper octant, to the signal and reference inputs of a conventional a/d converter to produce an output digital signal proportional to the tangent of the shaft angle and, if needed, recovering the angular information from the tangent signal. Octant selector proposals are discussed in the part in Electronic Design, 6.
Programmable limit switches use the digitized angle signal from a resolver (or synchro)-to-digital converter for control functions such as activating and deactivating limit switches depending on the shaft angle. The same digitized angle signal can be used for motion detection and tachometer functions, or for other control functions. Examples are referred to in manufacturer's brochures such as Mark VII Micro-Computer Programmable Limit Switches, 1982, C&A Products, Inc.; Microprocessor-Based Limit Programmer, undated, Sequential Information Systems, Inc.; Microcomputer PLS, September 1980, Gemco Electric.
Accuracy, reliability and low cost are some of the desirable characteristics of both resolver-to-digital converters and programmable limit switches, but are not believed to be successfully combined in the known prior art systems. Accordingly, this invention is directed to achieving this desirable combination of features in a particularly advantageous manner.
An exemplary and nonlimiting embodiment of this invention uses a resolver-to-digital converter in which the reference voltage for exciting the resolver input is derived from the status of a reference bit complemented periodically by a microprocessor programmed to generate periodic interrupts. The same microprocessor interrupts control the timing of the analog-to-digital conversion of the resolver outputs to ensure that the a/d cycle is in the most advantageous phase relationship with the reference voltage which excites the resolver rotor and is induced into the resolver stator windings. This improves accuracy as compared to known systems in this field, and reduces cost because it eliminates the requirement for separate reference voltage generators and peak detection circuits. In addition, the outputs of the resolver are subjected to fullwave rectification synchronized with the reference voltage to thereby substantially increase the average voltage level of the resolver outputs as compared with the known prior art, to thereby increase reliability and improve noise immunity.
Briefly, an exemplary and nonlimiting embodiment of the invention involves programming a microcomputer to generate periodic interrupts and to complement an output bit at each, and using the changes in the state of the output bit to generate a reference a-c voltage for exciting the rotor winding of the resolver. In response, the resolver provides a first output which is at the frequency of the reference voltage but is amplitude-modulated as a function of the sine of the shaft angle, and a second output which is similarly modulated as a function of the cosine of that angle. These outputs are fullwave rectified to the polarities of the sine and cosine envelopes in synchronism with the status changes in the output bit of the microcomputer, and the resulting enhanced voltage signals, after going through low pass filters, are used to find the first two bits determining the octant of the shaft angle, while their absolute values are compared with each other to find the third bit defining the octant, and to serve as inputs to a conversion cycle for finding a binary code representation of the tangent of the shaft angle within the current octant. This conversion cycle is short as compared to the cycle of the reference voltage and is synchronized with the output bit status changes such that it takes place in a selected phase relationship with the reference voltage, e.g. just before the positive peaks thereof. The 3-bit octant designator and the binary tangent representation are processed, e.g. through look-up tables, to give an absolute value for the current shaft angle. This absolute value can be offset and/or scaled as desired, and the result is compared with the from and to settings of limit switches to provide control signals commanding the activation or deactivation of the relevant limit switches in the relevant channels.